The widespread use of cardioversion and defibrillation has revealed a significant number of cases in which arrhythmias, acute myocardial failure, S-T elevation and other signs of cardiac injury have occurred following electric countershock. Morphological signs of injury have also been observed, but not studied in detail. The purpose of the proposed program is to study the ultrastructural and histochemical changes which occur under the influence of electrical stimuli to those used in clinical countershock, and to correlate these changes with aberrations in the contractile activity presently being studied in our laboratory, in order to provide insight into the sites and mechanisms of injury. This knowledge may suggest a modification in clinical procedures which will reduce the risk of cardiac injury while preserving the beneficial effects of countershock treatment.